From the past, a brazing method for aluminum alloy-assembled articles has adopted a process wherein an assembled article is formed with a brazing sheet composed of a core alloy (an alloy of Al—Mn series) both surfaces of which has been clad with a filler alloy of Al—Si series or one of the surfaces of which has been clad with a sacrificial material (an alloy of Al—Zn series) and the other surface of which has been clad with a filler alloy (an alloy of Al—Si series), and integrated in a furnace by heating the article at a temperature around 600° C. exceeding the melting point (liquidus temperature) of the filler alloy. According to this brazing method, a number of positions can be brazed at the same time. Then it is an established industrial method for manufacturing various products including a core for a heat exchanger, as shown for example in FIG. 1. This brazing method is classified to a vacuum brazing method wherein Mg is added to a filler alloy and it is heated in a vacuum furnace for brazing thereby an oxide film on the surface of Al is destroyed by evaporation of Mg and the getter function, and the NB method (CAB method) wherein non-corrosive flux is used and brazing is carried out in a furnace in nitrogen gas atmosphere.
The heat-exchanger core is constructed, as shown in FIG. 1, by a tube 1 for passing a cooling medium, a corrugated fin 2 for use in heat radiation (referred to hereinafter conveniently as fin) brazed between the tubes 1, a header pipe 3 and a pipe 4 for supplying or discharging a cooling medium, an end cap 5 blocking the upper end or the lower end of the header pipe 3, and side plates 6 fixing between the two header pipes 3. The aforesaid brazing sheet is used as the tube 1 and the tube 1 and the fin 2 are brazed.
In a brazing method using the aforesaid Al—Si series filler alloy, however, a plurality of assembled articles is set in a furnace and the whole of the individual assembled articles is maintained for 3 minutes at a temperature of about 600±5° C. As it takes much time for elevation of temperature especially from about 550° C., a time of 30 minutes to 1 hour is required for brazing of 1 cycle. For example, a heating time for brazing a core for a heat exchanger as shown in FIG. 1 required indeed 46 minutes per cycle as shown in FIG. 10, the following problems have arisen as the brazing temperature is as high as about 600° C.
Namely, (1) at the time of brazing, Zn in a sacrificial material (an alloy of Al—Zn series) constructing aluminum brazing sheet is diffused to a core alloy (an alloy of Al—Mn series) so that the anti-corrosive property of the core alloy has been deteriorated. (2) At the time of brazing, Mn deposition in the core alloy has been formed a solid solution again so that thermal conductivity of the core alloy is deteriorated to damage thermal characteristics of the heat exchanger. (3) Alloy elements such as Cu and Mg effective for enhancing strength allows the solidus temperature to drop lower than the brazing temperature so that they cannot be added to the core alloy. In addition, Mg is diffused onto the surface of the brazing sheet to deteriorate the function of flux at the time of brazing. (4) A recycled material of an aluminum heat exchanger contains 2 to 5 wt % of Si and depresses the solidus temperature to lower than the brazing temperature so that it is hardly used as core material.
Thus, the conventionally used filler alloy usable at high temperature involved various problems. However, it has been found that these problems can be solved by depressing the melting point of the filler alloy. Especially, if the brazing temperature can be depressed to about 540° C., melting of the filler alloy and the formation of fillet will rapidly be made to shorten the working time. Thus, filler alloy usable at low temperature composed of Al—Si—Zn—Cu alloy has been proposed (JP-A-3-57588 (“JP-A” means unexamined published Japanese patent application)). However, the filler alloy had a problem that it did not have a sufficient corrosion resistance.
Other and further features and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawings.